GROWTH SUBSTANCES FOR GEOTROPISM 



209 



+ 



be assumed with a fair degree of certainty that its accumulation 

 on the lower side of a horizontally placed root is the result of its 

 transverse movement. 



Mechanism of Growth-substance Displacement. — The geoelectric 

 phenomenon, whereby the lower side of the root is positively 

 charged with respect to the upper, through the action of gravity, 

 may be significant for the transverse displacement of the growth 

 hormone. Whether root curvatures 

 can be brought about by a potential 

 gradient has been investigated with 

 diverse results. Letellier (1899) studied 

 the effect of a zinc plate charged to 

 between 384 and 576 volts upon buried 

 roots of Vicia Faha. When the plate 

 was positively charged, a negative 

 curvature resulted; when negatively 

 charged, the curvature was not ob\4ous. 

 In the experiments of Brauner and 

 Biinning (1930) mentioned previously, 

 a strong curvature toward the negative 

 pole resulted in roots of Vicia Faha in a 

 field of continuous current of 640 volts 

 per centimeter (Fig. 63). If one as- 

 sumes that the displacement of growth 

 substance is brought about by a differ- 

 ence in electric potential, it should be field. When a root is placed 



expected that this displacement would "^l^"^^^^ electrodes in a moist 



^ _ _ '^ _ chamber, it bends toward the 



be in the direction of the induced negative electrode. Compare 



positive tissue. In the roots oi Pisum ^?J^.^^- ^^- }I'j°T ^'■°""''' 



'^ ana Bunnirig, 1930.) 



sativum, Hartmann (1932) observed 



curvatures in the direction opposite to those observed by Letellier, 

 though the same methods were employed. In Amlong's (1933) 

 experiments with roots of Vicia Faha, there was a good qualitative 

 and quantitative agreement in the first hour between the course 

 of the geotropic and electrotropic curvature, but later on the geo- 

 tropic curvature became by far the stronger. In evaluating these 

 investigations one comes to the same conclusion as that which 

 was reached for the Avena coleoptile, viz., that an electrostati- 

 cally produced potential difference is not sufficient to produce a 

 curvature of the same magnitude as that found in geotropic 



Fig. 63. — Curvature of a 

 Vicia Faha root in an electric 



